This invention relates to devices for the operation of drum brakes used for trucks or trailers and in particular with devices for the adjustment of the clearance between the brake lining and the drums.
In the adjustment device described in French Patent 2,277,274, a cam for adjusting the clearance of the brake shoes is driven by means of a shaft, lever and gear. The adjustment is effected by a helical gears driving the gear on the camshaft.
Devices for automatic clearance adjustment which attempt to progressively ratchet the cam at each operation of the brake are described in French Patent Nos. 2,389,042 and 2,224,676. In these devices, the brake shoes are kept against the drums for a certain length of time. This causes wear of the brake lining and an overheating of the drums, which can over a long period cause a blow-out of the tires.
French Patent Nos. 1,604,913 and 2,238,090 show ratcheting systems which insure, at each braking, a cancellation of the clearance, which may exist at rest between the brake lining and the drums, without thereby introducing a residual braking. However, this reduction of clearance does not permit the retrieving of the clearance, which is recommended in order to account especially for the successive expansions due to prolonged brakings. Also, these devices may produce certain wheel lock-ups, which are a detriment to the stability of the vehicle and to the life of the tires.